The present application applies to the measurement of the center detector in Computed Tomography (CT) equipment. While it applies to a variety of computed tomography equipment, it finds particular application to security examination equipment, such as baggage systems commonly found in airports.
Computed Tomography, in general, generates a three-dimensional image of an object from a series of two-dimensional measurements taken about a single axis of rotation. An object under examination is exposed to radiation, and images are formed based upon the radiation absorbed by the object, or rather an amount of radiation that is able to pass through the object. Highly dense objects absorb more radiation than less dense objects, and thus an object having a high density, such as a metal gun or bone, for example, will be apparent when surrounded by less dense objects, such as clothing or tissue.
In baggage systems, Computed Tomography equipment is used to detect weapons, explosives, and other prohibited items that may be contained in a bag or suitcase being scanned. One type of CT baggage scanner is described in U.S. Pat. No. 6,256,404 (Gordon et al.). In particular, an x-ray tube and a detector array are mounted on diametrically opposing sides of an annular shaped rotating platform, or disk, disposed within a gantry support for rotation about a single axis that is parallel to the direction of travel of the baggage (e.g., along a conveyor system). The x-ray tube emits x-rays, and the x-rays traverse the baggage under examination. X-rays that are not absorbed by the baggage and/or objects therein are detected by a detector. Data from the detector is used by reconstruction algorithms to create an image of the bag and the contents thereof.
The reconstruction algorithms rely on the value of a center detector to perform calculations that convert the data from the detectors into useful images. In order to mitigate artifacts (e.g., streaks at the edges of objects) on the image, the value of the center detector is periodically calibrated. One technique currently used to calibrate the center detector value is known as pin calibration, the details of which are described in G. T. Gullberg, B. M. W. Tsui, C. R. Crawford and E. Edgerton, “Estimation of geometrical parameters for fan beam tomography”, Physics in Medicine and Biology, 1987, Vol. 32, No. 12, pp. 1581-1592. On security scanners, pin calibration comprises stopping a conveyor belt and placing a cylindrical pin phantom (e.g., a test article of known, physical configuration) into the scanning field of the scanner (e.g., via a user who has to crawl into the scanner). Offset, air, and phantom data are acquired during an axial scan of the phantom. The data is analyzed to determine a center detector value. It will be appreciated that other phantoms, such as those described in U.S. Pat. No. 6,813,374 (Karimi et al.), may also be used for performing calibrations and tests on a scanner.
While current calibration techniques, such as pin calibration, have proven effective in determining the center detector value, there remains room for improvement. For example, the pin calibration technique requires a dedicated pin phantom with custom mounts. Additionally, a service technician has to crawl into the scanning field of the scanner to position the phantom and remove the phantom once the calibration procedure is over. Thus, the procedure is time consuming and cumbersome to execute because the scanner may not be easily accessible when integrated with entrance and exit conveyor tunnels, for example.